Biofilms are complex communities of microorganisms that are commonly found on a variety of substrates or surfaces that are moist or submerged (Musk et al., Curr. Med. Chem., 2006, 13, 2163; Donlan et al., Clin. Microbiol. Rev., 2002, 15, 167). Though primarily populated by bacteria, biofilms can also contain many different individual types of microorganisms, e.g., bacteria, archaea, protozoa and algae. The formation of biofilms can be thought of as a developmental process in which a few free-swimming (planktonic) bacteria adhere to a solid surface and, in response to appropriate signals, initiate the formation of a complex sessile microcolony existing as a community of bacteria and other organisms. Bacteria within biofilms are usually embedded within a matrix, which can consist of protein, polysaccharide, nucleic acids, or combinations of these macromolecules. The matrix is a critical feature of the biofilm that protects the inhabiting organisms from antiseptics, microbicides, and host cells. It has been estimated that bacteria within biofilms are upwards of 1,000-fold more resistant to conventional antibiotics (Rasmussen et al., Int. J. Med. Microbiol., 2006, 296, 149).
Biofilms play a significant role in infectious disease. It is estimated that biofilms account for between 50-80% of microbial infections in the body, and that the cost of these infections exceeds $1 billion annually. For example, persistent infections of indwelling medical devices remain a serious problem for patients, because eradication of these infections is virtually impossible. A few diseases in which biofilms have been implicated include endocarditis, otitis media, chronic prostatitis, periodontal disease, chronic urinary tract infections, and cystic fibrosis. The persistence of biofilm populations is linked to their inherent insensitivity to antiseptics, antibiotics, and other antimicrobial compounds or host cells.
Cystic fibrosis (CF), with 7 million asymptomatic heterozygous carriers, is one of the most common genetic diseases in the United States. Despite significant progress in the management of the symptoms of CF, virtually all CF patients succumb to chronic pulmonary infections. For reasons that are not entirely clear, the airways of CF patients are particularly susceptible to bacterial colonization. CF patients typically become infected with Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Burkholderia cepacia complex, and nonmucoid Pseudomonas aeruginosa. However, as the patients age, Pseudomona aeruginosa becomes the predominant pulmonary pathogen, present in up to 85% of cultures from patients with advanced disease. Once colonized by Pseudomonas aeruginosa, the organism persists for many years or decades and is never eradicated. This persistence of Pseudomonas aeruginosa has been linked to its ability to form biofilms. Complications arising from Pseudomonas aeruginosa infections are the leading cause of death among CF patents.
Deleterious effects of biofilms are also found in non-medical settings. For example, biofilms are a major problem in the shipping industry. Biofilms form on and promote the corrosion of ship hulls and also increase the roughness of the hulls, increasing the drag on the ships and thereby increasing fuel costs. The biofilms can also promote the attachment of larger living structures, such as barnacles, to the hull. Fuel can account for half of the cost of marine shipping, and the loss in fuel efficiency due to biofilm formation is substantial. One method of controlling biofilms is to simply scrape the films off of the hulls. However, this method is costly and time-consuming, and can promote the spread of troublesome non-native species in shipping waters. Another method involves the use of antifouling coatings containing tin. However, tin-based coatings are now disfavored due to toxicity concerns.
Given the breadth of detrimental effects caused by bacterial biofilms, there has been an effort to develop small molecules that will inhibit their formation (Musk et al., Curr. Med. Chem., 2006, 13, 2163). The underlying principle is that if bacteria can be maintained in the planktonic state, they will either not attach to a target surface and/or they can be killed by a lower dose of microbicide.
Despite the extent of biofilm driven problems, examples of structural scaffolds that inhibit biofilm formation are rare (Musk et al., Curr. Med. Chem., 2006, 13, 2163). The few known examples include the homoserine lactones (Geske et al., J. Am. Chem. Soc., 2005, 127, 12762), which are naturally-occurring bacterial signaling molecules that bacteria use in quorum sensing (Dong et al., J. Microbiol., 2005, 43, 101; Nealson et al., J. Bacteriol., 1970, 104, 313), brominated furanones isolated from the macroalga Delisea pulchra (Hentzer et al., Microbiology-Sgm, 2002, 148, 87), and ursene triterpenes from the plant Diospyros dendo (Hu et al., J. Nat. Prod., 2006, 69, 118). While the focus has predominantly been on designing small molecules that inhibit the formation of biofilms, one of the more significant challenges is the development of a small molecule that disperses pre-formed biofilms. None of the small molecules noted above have been previously reported to disperse an existing biofilm.
In addition, bacteria have an unparalleled ability to overcome foreign chemical insult. For example, resistance to vancomycin, “the antibiotic of last resort,” has become more prevalent, and strains of vancomycin-resistant Staphylococcus aureus have become a serious health risk. It has been predicted that it is simply a matter of time before different bacterial strains develop vancomycin resistance, and the safety net that vancomycin has provided for decades in antibiotic therapy will no longer be available. Therefore, the identification of chemical architectures useful to inhibit biofilm development is needed.
Because of their natural resistance to antibiotics, phagocytic cells, and other biocides, biofilms are difficult, if not impossible, to eradicate. Therefore, the identification of compounds that control biofilm formation is of critical need.